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The Tree of Life – How many species? 2 million, 10 million? 100 million? And it’s not really a tree.

The most familiar species, those of animals and plants, will take up only a tiny part of the tree. “Most biodiversity on earth is microbial,” said Dr. Katz, the biologist at Smith.

Microbes also pose a special challenge. The branches of the tree of life represent how organisms pass their genes to their descendants. But microbes also transfer genes among one another. Those transfers can join branches separated by billions of years of evolution.

“In a lot of the tree of life, it’s not really treelike,” Dr. Cranston said.

She and her colleagues are exploring how they can build their database to include these gene transfers, and how best to visualize them. “That’s an issue we intend to struggle with for the next three years,” Dr. Katz said.